1. Field of the Invention
The present invention relates to a magneto-optical pickup apparatus for recording/reproducing an information signal or from a magneto-optical recording medium by utilizing a magneto-optical effect such as a magnetic Kerr effect or a Faraday effect, and a magneto-optical recording/reproducing apparatus using the same.
2. Description of the Prior Art
Background art in order to record a signal on a magneto-optical recording medium, it is known to use a light modulation method or a magnetic field modulation method. Of these methods, the magnetic field modulation method is advantageous since it can achieve an overwrite operation.
FIG. 1 shows a conventional magneto-optical pickup apparatus in which the magnetic field modulation method is employed.
A magneto-optical pickup apparatus shown in FIG. 1 is arranged as follows. When a single-sided magneto-optical recording medium 50 is loaded, an optical system including a laser device 58, an objective lens 59 and the like is arranged on the upper surface side of the single-sided magneto-optical recording medium 50, and a magnetic field generator 60 is arranged on the lower surface side of the recording medium 50.
A driving system (not shown) is arranged to drive the optical system in focusing and tracking directions with respect to the magneto-optical recording medium 50. Another driving system (not shown) is arranged to drive the magnetic field generator 60 in the direction of an arrow in FIG. 1 and the tracking direction.
In the magnetic field modulation method, since a magnetic field must be reversed at high speed, a sufficient excitation current cannot be obtained, and the generated magnetic field strength is limited. For this reason, the magnetic field generator 60 is arranged near a recording magnetic layer 53 (to be described later) of the single-sided magneto-optical recording medium 50.
The single-sided magneto-optical recording medium 50 is constituted by sequentially stacking a dielectric layer 52, a recording magnetic layer 53 formed of, e.g., a rare earth-transition metal alloy amorphous thin film, and having a large magneto-optical effect, a dielectric layer 54, a reflecting layer 55 and a protection cover 56 on one surface of a transparent substrate 51 formed of, e.g., polycarbonate. In this case, the thickness of the transparent substrate 51 is constant, e.g., 1.2 mm.
An operation of the prior art device will be briefly described below.
The single-sided magneto-optical recording medium 50 is placed on a rotary disk (not shown), and a magnetic field is applied from the magnetic field generator 60 to the recording magnetic layer 53 of the single-sided magneto-optical recording medium 50. The magnetic field to be applied is reversed at high speed, and a laser beam emitted from the laser device 58 is focused on the recording magnetic layer 53 applied with the magnetic field, so that a region of the recording magnetic layer 53 where the laser beam is focused can be magnetized, and an information signal can be overwritten in real time.
Along with an increase in information volume in recent years, a double-sided magneto-optical recording medium which can record information signals on two surfaces has been developed.
As shown in FIG. 2, a conventional double-sided magneto-optical recording medium 70 is constituted by bonding a magneto-optical recording medium A obtained by sequentially stacking a transparent substrate 71a, a dielectric layer 72a, a recording magnetic layer 73a, a dielectric layer 74a and a reflecting layer 75a, and a magneto-optical recording medium B obtained by sequentially stacking a transparent substrate 71b, a dielectric layer 72b, a recording magnetic layer 73b, a dielectric layer 74b and a reflecting layer 75b by means of an adhesive 76. In this double-sided magneto-optical recording medium 70, the thickness of the transparent substrates 71a and 71b is several mm to assure a sufficient mechanical strength. Since the recording medium 70 is constituted by stacking the transparent substrates 71a and 71b, the recording magnetic layer 73a, and 73b and the like, the medium 70 becomes thicker than to the single-sided magneto-optical recording medium 50.
When the conventional magneto-optical pickup apparatus of the magnetic field modulation type shown in FIG. 1 employs the double-sided magneto-optical recording medium 70, the following problems arise. Since the optical system is arranged on the upper surface side of the double-sided magneto-optical recording medium 70 and the magnetic system is arranged on the lower surface side thereof to perform recording, if recording is performed on the upper magneto-optical recording medium A of the double-sided magneto-optical recording medium 70, a laser beam can be focused on the recording magnetic layer 73a from the upper surface side. But a magnetic field with a sufficient intensity cannot be applied from the magnetic field generator 60 on the lower surface side since the recording medium 70 itself is too thick. In the magnetic field modulation type magnetic field generator, a high-frequency current responding to a high-frequency data signal must be flowed through an electromagnetic coil. However, since the current cannot easily be flowed through the coil as the frequency is increased, the generated magnetic field is limited. In addition, the distance between the magnetic field generator 60 and the recording magnetic layer 73a is large. For this reason, it is very difficult to achieve double-sided magneto-optical recording of a magnetic field modulation method by the conventional techniques.